Photomask used by photolithography and a process of producing same

ABSTRACT

A photomask used by photolithography and a process for producing same which allows a single exposure to make a photomask, thereby simplifying the photomask making process, and facilitating the inspection and correction of photomasks. In addition, the phase shifter using a slanting pattern prevents a pattern from being formed outside a predetermined area. The use of a phase shifter which does not resolve under an optical projection system shields a large size area against an irradiated light, thereby allowing the formation of fine, intricate patterns suitable for use in LSIs.

This application is a division of application Ser. No. 08/081,835, filedJun. 24, 1993, which is a continuation of application Ser. No.07/718,337, filed Jun. 19, 1991, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photomask used by photolithography,and a process for making same.

2. Description of the Prior Art

In recent years, semiconductor technology has made remarkable progress.In line with this progress, highly intricate patterns have been requiredfor sophisticated semiconductor devices, such as LSIs. The formation ofintricate or highly fine patterns is entirely due to the recentdevelopment of lithography.

Lithography consists of a process in which a resist is first coated,then exposed to light and developed with a developer. The fineness ofthe resulting patterns has been improved by the development of thematerial of the resist, the device and method of exposure.

There are proposals for reducing the size of patterns, which, forexample, are disclosed in Japanese Patent Publication No. 62-50811 andNo. 62-59296. The feature common to these proposals is that a photomaskused for exposing radiation is improved so that a phase differencearises in the irradiated light or incident light upon the photomaskthrough an optical system, thereby achieving fine, intricate patterns.

Referring to FIGS. 26 and 27, the known photomask will be described ingreater detail:

A photomask substrate 1 is prepared, and a light-shielding layer 2 isformed thereon so as to give a circuit pattern to be transferred.Transparent or transmissive layers 3 are provided on either side of thelight-shielding layer 2. One of them acts as a phase shifter. The phaseshifter is to change the phase of an exposing radiation.

In FIG. 27, the substrate 1 is composed of two different members 4 and 5arranged side by side, of which the members 4 are transparent ortransmissive layer. The members 4 act as phase shifters. This example ischaracteristic in that the phase shifter 4 is not formed on thesubstrate 1. This type of photomask is disclosed in Japanese Laid-OpenPatent Publication No. 62-92438.

These prior art photomasks have the following disadvantages:

In the photomask shown in FIG. 26 the light-shielding pattern is formedin a conventional photomask making process; more specifically, asubstrate is wholly covered with a thin layer so as to form alight-shielding layer pattern. This thin layer is made of chromium layeror molybdic silicide layer. The thin layer is then coated with a resist,and baked. Then a desired area is exposed to light or electron beams.The resist in the exposed area is removed or remain by using anappropriate developer so as to form a resist pattern. Finally, a liquidor gaseous etchant is used, called wet etching or dry etching, to etchthe thin layer into a light-shielding pattern.

Then, a phase shifter 4 of a thin film is formed on the light-shieldingpattern and a transparent area outside the light-shielding area. Thethin layer is made of photosensitive resin. Light or electron beams areradiated on a desired area in the thin layer. It is necessary to alignthe exposing radiation with the edges of the light-shielding patterns.This means two times of exposure, thereby resulting in a complicatedexposing procedure. In addition, the alignment must be strictlyaccurate, otherwise a desired phase shifter will not be attained.

In general, a photomask must be perfect, free from any defects. However,it is difficult to detect any fault in the photomask by scanning lightso as to find a scattering light or a permeating light. This detectingmethod is likely to be inaccurate because of an out-of-focus irradiationcaused by the uneven thickness of the phase shifter due to the presenceof the light-shielding layer, and because of the impermeability of lightin the overlapping of the light-shielding layer over the phase shifter.In this case it is necessary to produce another phase shifter so as toremedy the defects.

In the phase shifter shown in FIG. 27 the phase shifters are arranged atequal intervals. In order to achieve a desired phase difference, it isnecessary to fabricate the intervals to a high precision, therebyresulting in a difficult fabrication of photomasks. Irregular intervalscannot produce fine, intricate patterns suitable for IDs used in LSIs.

SUMMARY OF THE INVENTION

The photomask of this invention, which overcomes the above-discussed andnumerous other disadvantages and deficiencies of the prior art,comprises a transparent substrate, and a transparent layer patternformed on the transparent substrate, the transparent layer patterngiving a phase difference to an incident light.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, whereinthe following relationship is established:

    t=λ/2(n-1)

where t is a thickness of the transparent layer pattern, λ is awavelength of an incident light, and n is an index of refraction.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, thetransparent layer pattern giving a phase difference of 180° multipliedby an odd number to an incident light.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, thetransparent layer pattern giving a phase difference to an incidentlight, wherein the intensity of an incident light is minimized at anedge of the transparent layer pattern.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, thetransparent layer pattern giving a phase difference of 180° multipliedby an odd number to an incident light, wherein the following equation isestablished:

    R=k.sub.1 ·λ/NA

where R is a limit for the resolution of an optical resist pattern, k₁is a constant having a value of about 0.35, λ is a wavelength of anincident light, and NA is a numerical aperture of an optical lens.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, and anarea which is shielded from light by the transparent layer.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, and anarea which is shielded from light by the transparent layer, the shieldedarea being formed by a pattern having a patch below the opticalresolution limit.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, and anarea which is shielded from light by the transparent layer, the shieldedarea comprising a repetition of patterns having a pitch below aresolution limit.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, and anarea which is shielded from light by the transparent layer, the shieldedarea comprising a repetition of patterns having a pitch below aresolution limit, wherein the following equation is established:

    R=k.sub.1 ·λ/NA

where R is a limit for the resolution of an optical resist pattern k₁ isa constant, λ is a wavelength of an incident light, NA is a numericalaperture of an optical lens.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, and anarea which is shielded from light by a repetition of patterns having apitch of not larger than four times the value of R.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, and anarea which is shielded from light by the transparent layer, the areaoccupying not larger than 10% of light intensity.

Alternatively, the photomask comprises a transparent substrate, and arepetition of transparent layer pattern formed on the transparentsubstrate, the pattern having a pitch below a resolution limit whichgives a phase difference to an incident light, and the pattern havinglight intensity in a zigzag form.

Alternatively, the photomask comprises a transparent substrate, and arepetition of transparent layer pattern formed on the transparentsubstrate so as to give a phase difference to an incident light, whereinthe pattern is repeated at a pitch in the range of 1.0 μm to 4.0 μm.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, thepattern comprising a repetition of line and space.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, thepattern comprising an matrix arrangement.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate, thepattern giving a phase difference to an incident light so as to form asingle resist pattern.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate so as togive a phase difference to an incident light, the pattern comprising alight-shielding part of a repetition of pattern having a pitch below aresolution limit, and a transparent part of a repetition of patternhaving a pitch above the resolution limit.

Alternatively, the photomask comprises a transparent substrate, and afirst transparent layer pattern forming a first resist pattern, and asecond transparent layer pattern formed reversely to the firsttransparent layer forming a second resist pattern, the first and secondresist pattern being identical in shape.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate so as togive a phase difference to an incident light, the transparent layerpattern comprising part of the periphery thereof slanting in thecross-section.

In a preferred embodiment, the transparent layer pattern comprises aslanting side thinning step by step.

In a preferred embodiment, the transparent layer pattern comprises aslanting side thinning continuously.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate so as togive a phase difference to an incident light, the transparent layerpattern comprising a repetition of projections having a pitch below aresolution limit.

In a preferred embodiment, the lengths of the projections determines thewidths of the resist pattern.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate so as togive a phase difference to an incident light, the transparent layerpattern having both ends slanting in the cross-section thereof.

In a preferred embodiment, the width of the slanting transparent patterncontrols the width of the widths of the resist pattern.

Alternatively, the photomask comprises a transparent substrate, and afirst transparent layer pattern formed on the transparent substrate soas to give a phase difference to an incident light, and a secondtransparent pattern formed on both sides of the first transparentpattern, the second transparent pattern having a pitch below aresolution limit.

In a preferred embodiment, the photomask is formed within a spaceconstituted by a plurality of a pitch of the second transparent layerpattern.

In a preferred embodiment, the widths of resist patterns are controlledby the width of an area in which the second transparent layer pattern.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer pattern formed on the transparent substrate so as togive a phase difference to an incident light, the transparent patterncomprising a repetition of pattern having a pitch below an optical phasedifference, and part of the periphery thereof having a slanting side inthe cross-section.

Alternatively, the photomask comprises a transparent substrate, and atransparent layer formed on the substrate, the transparent layercontaining a silicon-base resist pattern formed on the transparentsubstrate, the resist pattern containing siloxane polymer, and giving aphase difference of 180° multiplied by an odd number to the incidentlight.

According to another aspect of the present invention, there is a processfor forming a photomask, the process comprises the steps of coating atransparent substrate with a transparent layer to a sufficient thicknessto give a phase difference of 180° multiplied by an odd number to theincident light, and forming a transparent pattern by exposure to light.

Alternatively, the process for forming a photomask, the processcomprises the steps of depositing a transparent layer on a transparentsubstrate to a sufficient thickness to give a phase difference of 180°multiplied by an odd number to an incident light, coating thetransparent layer with a resist, forming a resist pattern by exposure tolight, and removing the transparent layer by the use of the resistpattern as a mask.

Alternatively, the process for forming a photomask, the processcomprises the steps of forming a resist pattern on a transparentsubstrate, depositing a transparent layer on the transparent substrateto a sufficient thickness to give a phase difference of 180° multipliedby an odd number to the incident light, removing the transparent layerby removing the resist pattern so as to enable a transparent pattern toremain on the substrate.

Alternatively, the process for forming a photomask, the processcomprises the steps of depositing a transparent layer on a transparentsubstrate to a sufficient thickness to give a phase difference of 180°multiplied by an odd number to the incident light, coating thetransparent layer with a resist, and exposing the resist to light bychanging the amount of exposure.

Alternatively, the process for forming a photomask, the processcomprises the steps of depositing a transparent layer on a transparentsubstrate to a sufficient thickness to give a phase difference of 180°multiplied by an odd number to the incident light, coating thetransparent layer with a resist, and effecting the exposure of part ofthe periphery of the resist by a light whose focus is deviated.

Thus, the invention described herein makes possible the objectives of(1) providing a photomask which simplifies the process of fabricatingphotomasks by reducing the number of exposures to one time, andeliminating the necessity of aligning the mask with a light-shieldinglayer, (2) providing a photomask which facilitates the inspection andcorrection of photomasks, (3) providing a photomask which enables theformation of highly fine, intricate patterns required for ICs of LSIs,and (4) providing a process for producing such photomasks.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects andadvantages will become apparent to those skilled in the art by referenceto the accompanying drawings as follows:

FIGS. 1(a) to 1(c) are diagrammatical cross-sections showing a photomaskaccording to the present invention, and FIG. 1(b) is a graph showing thedistribution of light intensity;

FIGS. 2(a) to 2(c) are diagrammatical cross-section showing a photomaskaccording to the present invention, and FIG. 2(b) is a graph showing thedistribution of light intensity;

FIGS. 3(a) to 3(f) are plan views showing various shapes of thephotomask according to the present invention;

FIGS. 4(a) to 4(c) are cross-sectional views showing various shapes ofmulti-stage phase shifters of the photomask according to the presentinvention;

FIGS. 5(a) to 5(c) are diagrammatical cross-section showing a photomaskhaving a multi-stage phase shifter, and FIG. 5(b) is a graph showing thedistribution of light intensity;

FIGS. 6(a) and 6(b) are plan views showing a photomask having amulti-stage phase shifters;

FIGS. 7(a) to 7(e) are plan views showing a photomask having other formsof multi-stage phase shifters;

FIGS. 8(a) and 8(b) are plan views showing a photomask having otherforms of multi-stage phase shifters;

FIGS. 9(a) to 9(c) are diagrammatical cross-sections showing a photomaskhaving a multi-stage phase shifter, and FIG. 9(b) is a graph showing thedistribution of light intensity;

FIGS. 10(a) to 10(c) are plan views of a photomask according to thepresent invention particularly exemplifying the formation of a blackoutportion;

FIG. 11 is a graph showing the relationship between the pitches andlight intensity;

FIGS. 12(a) and 12(b) are plan views showing an application of thephotomasks of FIG. 10;

FIGS. 13(a) to 13(e) are plan views showing various patterns formed onthe photomask to form the blackout portions;

FIG. 14(a) to 14(f) are plan views showing various widths of resistpatterns achieved by the photomask;

FIGS. 15(a) to 15(c) are views exemplifying the relationship between thephotomask according to the present invention and the widths of theresist patterns formed by the photomask;

FIGS. 16(a) and 16(b) are diagrammatic views exemplifying a process ofproducing of the photomask according to the present invention;

FIGS. 17(a) and 17(b) are diagrammatic views exemplifying anotherexample of the process of producing of the photomask according to thepresent invention;

FIGS. 18(a) and 18(b) are diagrammatic views showing chemical structuresof an electron beam resist for forming a phase shifter for the photomaskaccording to the present invention;

FIGS. 19(a) to 19(d) are diagrammatic views exemplifying other exampleof the process of producing the photomask according to the presentinvention;

FIGS. 20(a) to 20(d) are diagrammatic views exemplifying a furtherexample of the process of producing of the photomask according to thepresent invention;

FIGS. 21(a) to 21(j) are diagrammatic views exemplifying a furtherexample of the process of producing of the photomask according to thepresent invention;

FIGS. 22(a) and 22(b) are plan views showing the shape of a phaseshifter achieved when the photomask is applied to a semiconductorsubstrate;

FIGS. 23(a) and 23(b) are plan views showing another shape of the phaseshifter achieved when the photomask is applied to a semiconductorsubstrate;

FIGS. 24(a) and 24(b) are plan views showing another shape of the phaseshifter achieved when the photomask is applied to a semiconductorsubstrate;

FIGS. 25 is a plan view showing another shape of the phase shifterachieved when the photomask is applied to a semiconductor substrate;

FIG. 26 is a cross-sectional view exemplifying a known phase shifter;and

FIG. 27 is a plan view of other known phase shifter shown in FIG. 26.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a light transparent photomask substrate 1,hereinafter referred to merely as "substrate 1" is provided with a phaseshifter 2 which is formed by patterning a transparent layer. Nolight-shielding pattern is formed unlike the conventional photomasks.Hereinafter, the substrate 1 and the phase shifter 2 in combination arecalled the "transparent phase-shifter mask". FIG. 1(b) shows thedistribution of the intensity of a light passing through the photomaskwhen the latter is irradiated with a light. The "transparentphase-shifter mask" is intended to give a phase shift between the lightpassing through the pattern of the phase shifter 2 and the light passingthrough the substrate 1. The light passing through the phase shifter 2is shifted in an angular range from 90° to 270°. The phase differencecan be adjusted by selecting an appropriate thickness of the phaseshifter 2. Preferably, the layer is thick enough so as to ensure thatthe phase difference is 180° (π).

When a transparent phase-shifter mask having a phase difference of 180°is exposed to light, a light passing through the phase shifter 2 and alight passing through the substrate 1 have a phase difference of 180° toeach other, wherein the lights have no substantial phase difference(i.e. coherent lights). When coherent lights are introduced throughdifferent portions, they interact with each other. This interactionaffects the light width. If the phase difference is zero, which meansthat coherent lights passing through the space are mutually in conflict,they strengthen each other. As a result, the sum of the widths of thetwo lights causes an interference. In the transparent phase-shifter maska light passing through the substrate 1 and the phase shifter 2 have aphase difference of 180°. As a result, the two lights cause aninterference so as to weaken each other. A light passing through bothends of the phase shifter 2 causes an interference with a light passingthrough an adjacent substrate 1 because of their mutual phasedifferences being different by 180° so that the mutual light intensitiesare weakened. In other words, the intensity of light is minimizedbecause of the interference caused by the edges of the phase shifter 2.

FIG. 1(c) shows an example of forming a positive resist pattern by theuse of a photomask according to the present invention.

A semiconductor substrate 6 is prepared, on which a resist is coated toa thickness of 0.5 microns, and the coat is pre-baked so as to hardenthe resist. The hardened resist is exposed to an irradiation having theintensity shown in FIG. 1(b). The exposed portions are removed so as toform a resist pattern 7 with a developer.

The light intensity is minimized by every edge of the phase shifter 2.As a result, the number of the resist patterns 7 formed on thesemiconductor substrate 6 amounts to two times the number of thoseformed on the substrate 1. It will be understood from this thatresolving power is enhanced as compared with the known exposingtechnique utilizing no phase difference.

FIG. 2 shows an example of forming a single pattern (hereinafter called"single pattern") as a phase shifter 2. FIG. 2(a) is a sectional viewshowing a transparent phase-shifter mask according to the presentinvention:

The transparent phase-shifter mask is also made of the substrate 1 andthe phase shifter 2, having no light-shielding layer.

The single pattern is formed at a sufficient distance from otherpatterns formed on the substrate 1. Another pattern is also shown whichis free from an adverse influence when a resist pattern is formed, evenif the light is interfered by a light passing through other patterns onthe substrate 1.

As described with respect to FIG. 1, the lights passing through thephase shifter 2 and the substrate 1 have a phase difference. Two lightspassing through the edges of the phase shifter 2 have a phase differenceof 180°, thereby causing an interference which weakens the intensity.

As shown in FIG. 2(b), the distribution of light intensity on a wafer isminimized at the edges of the pattern. When this transparentphase-shifter is used to form a positive resist pattern, the resistpattern 7 takes the form shown in FIG. 2(c). In the case of a singlepattern formed with a light-shielding layer, the area covered with thelight-shielding layer remains as a resist pattern.

The transparent phase-shifter mask has no distinct areas of white (anarea through which a light passes) and of black (the area to which alight is shielded). The transparent phase-shifter mask allows a lightpassing through the substrate 1 to pass through, and a light passingthrough the phase shifter 2 to pass through. The present inventionrequires a light-shielding layer but utilizes the intensity of lightvarying owing to interference.

FIG. 3 shows shapes of a phase shifter 2 formed on the substrate 1. Whenthe single square pattern shown in FIGS. 3(a) and 3(b) is used, theresulting positive resist pattern takes the form shown in FIG. 3(c).

FIG. 3(a) shows a square area given by the phase shifter 2, and FIG.3(b) shows a larger area given by the phase shifter 2. By using eithermask, the same resist pattern 7 is formed as shown in FIG. 3(c) byirradiating a laser beam upon the semiconductor substrate 6. In thetransparent phase-shifter mask the resist pattern is formed at an areawhere the phases of the irradiated light differ by 180°. The intensityof light is reduced at this area, and after development, the resistaround this area remains. For these reasons, in the transparentphase-shifter mask of FIGS. 3(a) and 3(b), the edge portion of thesquare phase shifter 2, and other edge portions than the squarephase-shifter correspond to the area where the phases are displaced by180°. Thus, in each transparent phase-shifter mask the same shape ofresist patterns 7 are formed.

When exposure is made by the use of a single rectangular pattern asshown in FIGS. 3(d) and 3(e), the positive resist pattern shown in FIG.3(f) is formed.

FIG. 3(d) shows an area formed by a rectangular phase shifter 2 on thesubstrate 1 and FIG. 3(e) shows an area other than the rectangularportion which is formed by the phase shifter 2.

Both of these show the same resist pattern 7 as shown in FIG. 3(f). Thiswill be understood from the same explanation given with respect to FIGS.3(a) and 3(b).

More specifically, in transparent phase-shifter mask an area where thephase of a light is different by 180° has a least light intensity, andallows a resist around this area to remain after development. For thisreason, in the transparent phase-shifter mask of FIGS. 3(d) and 3(e)this area corresponds to the edge portions of the rectangular phaseshifter 2, and in FIG. 3(e) the area corresponds to the edge portions ofother than the rectangular phase shifter 2. As a result, in eachtransparent phase-shifter mask the same resist pattern 7 is formed.

The fine pattern shown in FIG. 3(c) is difficult to fabricate by the useof a photomask in common use or a conventional phase shifter mask. Thatis, if in photomasks made from a conventional light-shielding layer aresist pattern 7 to be formed is rectangular or square, thelight-shielding layer must be correspondingly rectangular or square. Inthis case, electron beams are used to pattern the resist coated on thelight-shielding layer. However, a problem arises in that when therectangular shape to be formed is extremely fine, the area of exposureis likely to expand owing to scattering of electron beams. As a result,the four corners of both rectangles are rounded. When a much finerpattern is to be formed, the square of the resist pattern 7 becomesrounded.

When a photomask formed with a rectangular light-shielding layer by theuse of electron beams is used to form a resist pattern, the desired areais exposed to light by diffraction through the edges of the shieldinglayer; more specifically, the corners of the square of the resistpattern 7 are rounded or the square remains.

Even in the conventional phase-shifter mask the same problem arisesbecause the light-shielding layer is formed and a phase shifter isformed in the edge portion. In contrast, in transparent phase-shiftermask a phase shifter a resist pattern can be formed in a desired shape.

A resist pattern 7 formed by the use of a transparent phase-shifter maskis formed along the periphery of a pattern of the phase shifter 2. Thisshapes the phase shifter 2 like an closed ring or alternatively aslightly modified shape. With the use of such shapes of resist patterns7 alone it is difficult to fabricate intricate patterns such as LSIs,and it is required to prepare various shapes other than the closed ringshape.

In order to meet such demands, another example has been provided, whichis characterized by the phase shifter which diminishes in thicknesscontinuously or step by step. Hereinafter, this type of phase shifter iscalled a multi-stage phase shifter, which will be described in detail:

Referring to FIG. 4, the photomask comprises a transparent photomasksubstrate 1, hereinafter called "substrate", and a phase shifter 2 madeof a transparent layer, having no light-shield pattern unlike theconventional photomasks.

The phase shifter 2 includes a multi-stage phase shifter 8 which has aleft-hand wall constantly upright to the main surface of the substrate 1and has a right-hand wall gradually changeable in thickness.

FIG. 4(a) shows the multi-stage phase shifter portion 8 whose one sidediminishes toward its end, and FIG. 4(b) shows the phase-shifter whoseone side diminishes in thickness step by step; in the illustratedembodiment, the thickness in the step diminishes by 1/2 of the entirethickness. FIG. 4(c) shows other multi-stage phase shifter 8 whose oneside has three steps, diminishing in thickness by 1/4 of the entirethickness.

The phase of an incident light is 180° at the left end of the phaseshifter 2, and makes a phase difference of 180° from the phase of anincident light to an area having no phase shifter. The incident light tothe right end of the multi-stage phase shifter 8 has a phase differencevarying from 180° to 0° (zero) continuously. In FIG. 3(b) the phasedifference of light is 180° in the phase shifter 2, 90° in themulti-stage phase shifter 8, and 0° in the photomask.

FIG. 4(c) shows an intermediate mode between those of FIGS. 4(a) and4(b), in which the phase difference of the incident light varies from180° to 0° step by step.

FIG. 5 shows a sectional structure of transparent phase-shifter maskhaving a multi-stage phase shifter, and an exposure theory.

FIG. 5(a) shows a multi-stage phase shifter 8 having two steps whichdiminishes by 1/3 of the entire thickness step by step. In this phaseshifter 2 the phase difference of light is 180° in the phase shifter 2,diminishes to 120° or to 60° in the multi-stage phase shifter 8, and to0° in the multi-stage phase shifter.

FIG. 5(b) shows the distribution of the intensity of light passingthrough the mask when it is irradiated with the use of a transparentphase-shifter mask having this type of multi-stage phase shifter 8.

The radiated light has a phase difference of 180° at the left endportion of the phase shifter 2, and its intensity is remarkably reduced.In the multi-stage phase shifter 8 the phase difference graduallydiminishes, and the reduction of light intensity due to interference isminimized. Thus, when such a transparent phase-shifter mask is used toform a positive resist pattern, a single resist pattern is formed asshown in FIG. 5(c).

FIG. 6 shows the shape of a phase shifter formed on the main surface ofthe substrate 1, and a resist pattern formed on a semiconductorsubstrate.

FIG. 6(a) shows a rectangular phase shifter having a multi-stage phaseshifter. This type of transparent phase-shifter mask has a phase shifterwhich gives a phase difference of 180° to the substrate 1. The phaseshifter 2 has a rectangular shape, and forms a rectangular shape ofresist pattern as shown in FIG. 3. The phase shifter includes a phaseshifter 2 and a multi-stage phase shifter 8. This multi-stage phaseshifter 8 is provided by the edges corresponding to the phase shifter 2that does not form a desired resist pattern. This multi-stage phaseshifter 8 is formed along the three edges of the phase shifter 2, andthe light intensity in the area enclosed by the multi-stage phaseshifters 8 is least reduced, thereby forming no pattern.

FIG. 6(b) shows a positive resist pattern formed by the use of this typeof transparent phase-shifter mask.

In this way the single resist pattern 7 is formed. If this single resistis to be formed by the use of a conventional photomask, the light willbe diffused to area other than a predetermined one under the influenceof diffraction. This prevents the resist pattern 7 from being formed ina desired form.

The conventional phase shifter mask is suitable for fabricating repeatedpatterns but not suitable for fabricating a single pattern. In contrast,the use of a transparent phase shifter mask enables a single resistpattern to be fabricated easily. In addition, a multi-stage phaseshifter can be used which changes the phase continuously or step bystep, thereby allowing the resist patterns to be formed at variousplaces.

FIGS. 7(a) to (d) shows the phase shift pattern of a photomask accordingto the present invention when it is used to form a resist pattern for aline-and-space.

FIG. 7(a) shows a phase shifter 2 which gives a phase difference of 180°to the transparent substrate 1. The phase shifter 2 is generallyrectangular, which forms a rectangular resist pattern as shown in FIG.3. In order to form single patterns, the phase shifter includes a phaseshifter 2 and a multi-stage phase shifter 8. In order to make a line andspace have a line and space, the resist pattern, the multi-stage phaseshifter 8 is provided at the top portion of the phase shifter 2.

In order to change the width of the space in the line and space, thewidths of a plurality of phase shifters 2 and the widths of a spaceexisting between the phase shifters 2 can be changed.

FIG. 7(b) shows a phase shifter 2 which gives a phase difference of 180°to a transparent photomask substrate 1. This phase shifter 2 isgenerally rectangular, which forms a rectangular resist pattern as shownin FIG. 3. In order to form a plurality of single patterns, the phaseshifter includes the phase shifter 2 and a multi-stage shifter 8. Inorder to make the resist pattern have a line and space, the multi-stagephase shifter 8 is provided at the top portion of the phase shifter 2.

In order to change the widths of a line and space of a resulting resistpattern, the widths of a plurality of phase shifters 2 and the width ofan area existing between the phase shifters 2.

FIG. 7(c) shows a reversed structure in which the phase shifter 2 andthe photomask substrate 1 shown in FIG. 7(a) are turned upside down asshown in FIG. 3. This structure enables the resist pattern to be formedin an identical shape to that of FIG. 7(a).

Likewise, FIG. 7(d) shows a reversed structure in which the phaseshifter 2 and the photomask substrate 1 shown in FIG. 7(b) are turnedupside down as shown in FIG. 3. This structure enables the resistpattern to be formed in an identical shape to that of FIG. 7(b).

FIG. 7(e) shows a resist pattern 7 formed by radiating light by the useof a transparent phase-shifter mask.

FIG. 8 shows a photomask and a resist pattern used for forming a linepattern in a matrix. FIG. 8(a) shows a phase shifter 2 which gives aphase difference of 180° to the transparent substrate 1. The phaseshifter 2 is generally rectangular, which forms a rectangular resistpattern as shown in FIG. 3. In order to form a plurality of resistpatterns in a matrix, the phase shifter includes the phase shifter 2 anda multi-stage phase shifter 8, wherein in addition to the phase shifter2 and the multi-stage phase shifter 8, a further multi-stage phaseshifter 8 is provided in a predetermined area in the phase shifter 2.

In this way the resist pattern in a matrix is formed as shown in FIG.8(b). If the lateral width of the line pattern is to be changed, it ispossible to change the lateral width of the plurality of phase shifters2 and the area existing between the phase shifters 2. If thelongitudinal width of the line pattern is to be changed, it is possibleto change the vertical thickness of the plurality of multi-stage phaseshifters 8.

Preferably, the cross-sectional structure is formed as shown in FIG.4(a) which diminishes in thickness continuously. However, in fabricatinga photomask the structure shown in FIG. 4(b), this structureadvantageously allows the amount of exposure to be relatively low tosuch an extent that the thickness of the multi-stage phase shifter 8 bereduced by half when a pattern of the phase shifter 2 is formed byexposure and development.

Referring to FIG. 9, a method for fabricating a large-size resistpattern by the use of a transmissive phase-shifter mask:

FIG. 9(a) shows a cross-section of a phase shift mask used forfabricating a large-size resist pattern. A transparent photomasksubstrate 1, hereinafter called "substrate 1" is provided with a phaseshifter 2 which is formed by patterning a transparent layer. Nolight-shielding pattern is formed unlike the conventional photomasks.

It depends upon the value of the pitch p of a pattern of the phaseshifter 2 whether the resist pattern is formed by the use of a phaseshifter 2. More specifically, when the pitch p is sufficiently wide, thelight through a transparent phase-shifter mask is least intensified atthe edge portion of the pattern of a particular phase shifter 2, and thelight intensity is most reduced at an edge portion of the phase shifter2 at edge portions of a phase shifter adjacent to the first phaseshifter 2. If an area between the two phase shifters 2 has a lighthaving an intensity above a particular level, a line and space patternis formed at the time of development. This particular level means alevel at which an optical resolving is possible. In this way therepeated pattern is formed as shown in FIG. 1.

If the pitch p is finer than the degree at which optical resolution(hereinafter referred to as "resolution") is possible, the distributionof light intensity on the substrate 1 lacks contrast, thereby failing toform a resist pattern. The lights passing through the pattern of thephase shifter 2 and the substrate 1 interfere with each other therebyweakening each other. As a result, the distribution of light will beunderstood from FIG. 9(b) that at the edge portion of the phase shifter2 having a wide pitch p, the light is least intensified, and the edgeportion of the phase shifter 2 adjacent to the first phase shifter 2 isleast intensified. In an area existing between the respective phaseshifters the light is intensified, but before it exceeds a predeterminedlevel, the light intensity is reduced by the edges of the phase shifter2. This process is repeated, and the light is intensified at an areawhere the pitch p becomes wider than the predetermined width. As isevident from the foregoing, the light intensity in a resist pattern fora wider resist pattern.

FIG. 9(c) shows a resist pattern 7 formed on the semiconductor substrate6 by the use of this type of transparent phase-shifter mask.

FIG. 10 shows a phase shifter formed on the photomask substrate so as toform a square resist pattern having a large area.

FIG. 10(a) shows a transparent phase-shifter mask having a rectangularpattern of a phase shifter 2 square phase shifter 2 arranged in amatrix, wherein the pitch p of the pattern of the phase shifter 2 havinga width not exceeding the limit of resolution in an optical exposuresystem.

FIG. 10(b) shows a transparent phase-shifter mask having aline-and-space pattern of a phase shifter 2 square phase shifter 2arranged in a matrix, wherein the pitch p of the pattern of the phasematrix, wherein the pitch p of the pattern of the phase shifter 2 havinga width not exceeding the limit of resolution in an optical exposuresystem.

FIG. 10(c) shows a square resist pattern 7 having a large area formed onthe semiconductor substrate 6 by exposure and development by the use ofa transparent phase-shifter mask.

The resolution of an optical resist pattern has a limit R, which isrepresented by the following formula:

    R=k.sub.1 ·λ/NA                            (1)

where k₁ is a constant, λ is a wavelength of light, and NA is anumerical aperture of the optical lens.

When a conventional photomask is used, k₁ is 0.5. When a conventionalshift mask is used, k₁ is 0.35.

Resolution limits R are compared between when a phase shift is used andwhen a conventional photomask is used, and it is found that the phaseshift mask has a resolution limit R reduced 1.4 times that of aconventional photomask, provided that other conditions than the value k₁are the same. As a result, the resulting resist pattern has a reducedwidth. In this specification "resolution limit" is under an opticalprojection system unless specified to the contrary.

When a transparent phase-shifter mask according to the present inventionwas experimentally used, the value of k₁ was about 0.35. This is thesame value obtained by the use of the conventional phase shift. This isrational in that the underlying theory of irradiation exposure is thesame for forming resist patterns between the use of a conventional phaseshift and a transparent phase-shifter mask of the present invention.More specifically, when the phase difference is 180° between a lightradiated by the edge of the phase shifter and a light radiated upon aphotomask substrate, the light intensity is minimized. When thisphenomenon is utilized so as to form a resist pattern, the value of k₁(resolution limit R) was about 0.35.

In the transparent phase-shifter mask the edge portion of the phaseshifter forms a resist pattern. A number of resist patterns amounts totwice that of the patterns of the phase shifter. The pitch p of thephase shifter patterns calculated from the width of the resist patternon the semiconductor substrate is R×4.

If the pitch p is above a resolution limit, the resist pattern will beresolved. In order to form a resist pattern for a large-size area, thepitch of a repeated phase shifter pattern must be not larger than fourtimes the value R.

Optically, the smaller this pitch p is, the higher the light-shieldingpower becomes. This is a desirable phenomenon. However, in the photomaskmaking process an excessively tiny pattern does not favorably affect theresults. When i is a source of light at the time of irradiationexposure, NA is 0.45, and a reducing exposing device of 5:1 is employed,the pitch p is preferably in a range of 1.0 to 4.9 μm, wherein the pitchof the resist pattern on the semiconductor substrate is in the range of0.2 to 0.6 μm.

FIG. 11 shows the results of the light-shielding performance of therepeated patterns of the phase shifter. The Y-axis indicates a lightintensity ratio where I is the light intensity achieved when a lighthaving an intensity of 10 is passed through the transparentphase-shifter mask, and the X-axis indicates the pitch p of the phaseshifter pattern. White circles indicate the result of the transparentphase-shifter mask when the phase shift pattern are arranged in aline-and-space, and black circles indicate the result of transparentphase-shifter mask when the phase shifter patterns are repeated in amatrix.

When the phase shifter takes a line-and-space form, the pitch p is inthe range of 1.0 to 3.0 μm, the light intensity ratio is within 10%.When the phase shifter pattern is in a matrix, the light intensity iswithin 10% if the pitch p is in the range of 1.0 to 4.0 μm. In FIG. 11,irrespective of increases in the pitch p the light intensity increases,which indicates that the phase shifter is not shaped as desired.

In order to form a resist pattern for a large-scale area, the phaseshifter having a line-and-space pattern can be used on condition thatthe pitches p are in the range of 1.0 to 3.0 μm, and the phase shifterhaving a matrix pattern can be used on condition that the pitches p arein the range of 1.0 to 4.0 μm.

By using a repetition of phase shifter patterns having a resolutionlimit, various patterns can be formed.

FIG. 12 shows a resist pattern a photomask designed to form a resistpattern in a single form by irradiation exposure and development. FIG.12(a) shows transparent phase-shifter masks disposed in a line-and-spaceshape at pitches below a resolution limit. A space is provided at aninterval I in the phase shifter 2.

As shown in FIG. 12(b), a resist pattern 7 is formed because this areais shielded against an irradiated light, and the resist in the spacehaving the interval I is exposed to an irradiated light passing throughthe space. In this way the resist pattern 7 has a void portion.

FIGS. 13(a) to (d) show a variety of transparent phase-shifter maskswhich allow resist patterns to become a hole pattern. FIG. 13(a) showspatterns of the phase shifter 2 arranged in a matrix with a pitch p,which is below a resolution limit. One phase shifter 2 in the center isremoved, thereby making the entire shape like a cross through which anirradiated light is passed. This cross portion allows an irradiatedlight to pass through because the pitch is larger than p; morespecifically, when an irradiated light passes through the phase shifter2 and an irradiated light passes through a portion of the photomasksubstrate 1 which is adjacent to the phase shifter 2, they cancel eachother out if their phases are reversed by 180°. If the mutualcancellation occurs, an irradiated light passes through a portion of thephase shifter 2 where the frequency of the light is disturbed, andcontributes to exposure, as shown in FIG. 13(a).

FIG. 13(b) shows patterns of the phase shifter 2 arranged at pitches pon the substrate 1, wherein the pitch p is below a resolution limit.There is provided a single phase shifter 2. As a result, the phaseshifter 2 takes the form of a cross, which allows an irradiated light topass through because the pitch is larger than p. Here again, when anirradiated light passes through the phase shifter 2 and an irradiatedlight passes through a portion of the photomask substrate 1 which isadjacent to the phase shifter 2, they cancel each other out if theirphases are reversed by 180°. If the mutual cancellation occurs, anirradiated light passes through a portion of the phase shifter 2 wherethe frequency of the light is disturbed, and contributes to exposure.

FIG. 13(c) shows a line-and-space pattern formed on the substrate 1,with a pitch p being below a resolution limit. The central portion takesthe shape of the letter H, where an irradiated light is passed throughbecause the pitch is larger than p.

FIG. 13(d) shows a line-and-space pattern formed on the substrate 1,with a pitch p being below a resolution limit. The central portion isvoid. The void portion takes the shape of the letter H, where anirradiated light is passed through because the pitch is larger than p.

FIG. 13(e) shows a resist pattern 7 formed on the semiconductorsubstrate 6 by radiating a light upon the transparent phase-shiftermask. The transparent phase-shifter mask allows part of a light to passthrough and makes it contribute to exposure. Ideally, a square resistpattern is formed but in the case of intricate patterns a single resistpattern having a circular void.

As described above, the repetition of intricate phase shifter patternsfunction as a light-shielding layer such as a chromium layer andmolybdic silicide layer.

In this way, by using a transparent phase-shifter mask to form alight-shielding area all patterns can be formed without the use of theconventional photomasks. In addition, intricate resist patterns areformed along the periphery of a phase shifter pattern, which means thatthe width of the resist pattern is fixed under the optical system anddifficult to adjust. However, in order to produce LSIs the width of themmust be accurately controlled. To this end, it is necessary todistinctly arrange a light-shielding area and a transparent area, theformer consisting of a phase shifter portion having a pitch p below aresolution limit, and the latter allowing an irradiated light to passthrough. Thus, the widths of a pattern whose dimension is a degreecapable of being resolved by the conventional photomask can becontrolled. For example, the interval I of the void area of the phaseshifter shown in FIG. 12 can be changed so as to control the resultingresist pattern. However, this cannot be applied to a case shown in FIG.1 where extremely intricate line-and-space pattern.

A method of controlling the widths of intricate patterns by the use oftransparent phase-shifter mask will be described:

FIG. 14(a) shows a phase shifter 2 formed in stripes on the substrate 1.Unless the widths of the individual strips of the phase shifter 2 andareas of the substrate 1 that are exposed between the stripes are belowa resolution limit, a resist pattern 7 is likely to be formed on asemiconductor substrate 6 as shown in FIG. 14(b) since the exposure ismade inadequate because of an decrease in the light intensity caused byedge portions of the phase shifter 2. This indicates that the edgeportions of the phase shifter 2 contribute to the exposure of the edgeportions of the phase shifter. As a result, the number of the stripes ofthe resist pattern amounts to be two times that of the phase shifter.

FIG. 14(c) shows a photomask having a first and a second phase shifters2 crossing each other formed on the substrate 1, wherein the shape ofthe individual stripes are the same as those shown in shifter are notcontinuous. More specifically, the stripes of the second phase shiftercross those of the first phase shifter at constant intervals. In otherwords, each stripe of the first phase shifter 2 has equally spacedprojections in opposite directions. These projections act aslight-shielders against an irradiated light. The resulting patterns haveslightly widened stripes as shown in FIG. 14(d), wherein the letters l,m and n indicate the width of each projection, the projecting length,and 1/2 (l+m), respectively.

It is necessary to keep the value of l below a resolution limit but notnecessary to keep the value of m below the resolution limit because itdecides the widths of a resist pattern to be projected. The widths ofthe resist pattern 7 to be transferred are virtually equal to 2n. Thearea having a width below a resolution limit acts as a light-shielderagainst an irradiated light. This means that the width of the resultingresist pattern does not depend upon the value of l but on the value ofm. The larger the value of m is, the wider the light-shielder becomes,which means that the width of the resist pattern 7 to be transferreddepends upon the length m of the projections.

The light intensity decreases along a sine curve. This is because theresist pattern 7 has a smaller width than the length of the projections.

FIG. 15(a) shows the sizes of the phase shifter 2, which has a width of0.8 μm. This size is equal to 2n, wherein the n is shown in FIG. 14(c).The width l of the phase shifter 2 shown in FIG. 14(c) is set to 0.2 μmso that the resulting pattern width may be below a resolution limit. Theprojections have a length of m.

FIG. 15(b) shows a cross-section of a resist pattern formed by exposureand development by using a transparent phase-shifter mask shown in FIG.15(a). The distance between the resulting resist patterns is equal to2n. The width of the resist pattern means a width w of the bottom of theresist pattern kept in contact with the semiconductor substrate 6 asshown in FIG. 15(b).

In FIG. 15(c) white circles indicate the results obtained when apositive resist is used, where an area irradiated with light is removedby development. As a result, the resist in a non-exposure area remainsas a resist pattern. Black circles indicate the results obtained when anegative resist is used, where an area not irradiated with light isremoved by development. As a result, the resist in an irradiated arearemains as a resist pattern.

When a positive resist is used, the width w becomes wider in proportionto the length m of the projection. When the length m of the projectionsis zero, that is, the case of a stripe-shape phase shifter, the width ofthe resulting resist pattern becomes about 0.25 μm. When the length m is0.4 μm, the width of the resulting resist pattern becomes 0.4 μm. Thus,a line-and-space pattern is formed at 1:1.

When a negative resist is used, the resist pattern becomes narrower ininverse proportion to the length of the projections. When the length mof the projections is zero, that is, the case of a stripe-shape phaseshifter, the width of the resulting resist pattern becomes about 0.48μm. When the length m of the projections is 0.28 μm, the width w of theresulting resist pattern becomes 0.4 μm. Thus, a line-and-space patternis formed at 1:1.

In this way, the width of the resist pattern to be transferred byprojection and exposure is virtually equal to 2n. The area of a phaseshifter having a width below a resolution limit acts as alight-shielding layer against an irradiated light. This indicates thatthe width of the resulting resist pattern does not depend upon the widthl of the phase shifter but upon the length m of the projections. Thelight-shielding area becomes wider in proportion to the length of theprojections. The width of a resist pattern to be transferred dependsupon the length m of the projections.

FIG. 14(e) shows a phase shifter 2 formed in stripes on the substrate 1of FIG. 14(a), wherein the phase shifter 2 is provided with amulti-stage phase shifter 8 along the opposite sides thereof. m and nindicate the width of the multi-stage phase shifter 8 and 1/2 (the widthof the stripe-shape phase shifter 2+m). The value m decides the width ofthe resist pattern 7 to be projected. The width of the resist pattern 7to be transferred by projection and exposure is virtually equal to 2n.Thus, the resulting resist pattern 7 becomes thicker in proportion tothe value m. The width is equal to the sum of the width of thestripe-shaped phase shifter 2 and m. This means that the width of theresist pattern 7 can be controlled by changing the value of m. Thereason why the width of the resist pattern 7 does not exceed the sum of2m and the width of the stripe-shaped phase shifter 2 is that the lightintensity decreases at a sine curve by the edge portions of theprojections. As a result, the resist pattern 7 formed by developing anirradiated area is short of the terminating ends of the projections. Thelight-shielding area becomes wider and results in the pattern shown inFIG. 14(d).

FIG. 14(f) shows a photomask having a stripe-shaped inner phase shifter2a on the substrate 1 of FIG. 14(a). This inner phase shifter 2a isprovided with outer phase shifters 2b along the opposite sides of theinner phase shifter 2a. l, n and m indicate a distance between thecenters of the inner phase shifter 2a and the outer phase shifter 2b,the width of the outer phase shifter 2b and an interval between theinner phase shifter 2a and the outer phase shifter 2b, wherein the m andn are below a resolution limit. The width of the resist pattern 7 to betransferred by projection and exposure is virtually equal to 2l.

Because of the area acting as a light-shielding layer, the width of theresist pattern 7 depends upon the sum of the width n of the outer phaseshifter 2b and the interval between the inner and outer phase shifters2a and 2b. The light-shielding area becomes wider in proportion to thewidth n of the outer phase shifter 2b and/or the interval between theinner and outer phase shifters 2a and 2b, provided that both values of mand n are below a resolution limit. If the sum of m and n are set to anexcessively large value, either of them will come to exceed theresolution limit. In such cases, it is preferable that the patternconsisting of the inner and outer phase shifters 2a and 2b withintervals therebetween should be repeated.

The resulting pattern takes the form shown in FIG. 14(d).

As is evident from the description with respect to FIGS. 14(a), 14(c),14(e) and 14(f), intricate patterns can be formed by controlling thewidths by the use of the patterns shown therein.

A process of fabricating the transparent phase-shifter mask will bedescribed by way of examples:

A first example will be described by reference to FIG. 16(a). A quartzsubstrate 1 is prepared, and a transparent electroconductive layer suchas indium tin oxide (hereinafter called ITO layer) is deposited to athickness of 0.1 μm on the substrate 1. Then, a transparent layer 9 isspin-coated on the substrate 1 as a phase shifter 2. For the transparentlayer 9, polymethyl methacrylate (hereinafter called PMMA layer) isspin-coated to a thickness of 0.372 μm. Then, an exposing radiation isapplied through electron beams 10 at an accelerating voltage 25 KV bythe amount of exposure of about 130 μC/cm². The radiated surface issprayed with a mixture methylisobutyl ketone (hereinafter called MIBK)and isopropyl alcohol (hereinafter called IPA) at a ratio of 1:2 forthree minutes so as to effect development. The development dissolves theexposed part to form a desired pattern. Then, the substrate 1 is rinsedwith IPA, and spin-dried. In this way the transparent phase-shifter maskof FIG. 16(b) is formed.

Preferably, the substrate 1 is covered with an ITO layer to form adesired pattern of PMMA. This ITO layer of electroconductivity iseffective to prevent electrons from accumulating in the substrate 1which is inherently an insulator allowing the accumulation of electronsin the substrate 1. The accumulated electrons are likely to disturb theelectron beams 10, thereby distorting a resulting pattern. The ITO layercan be only thick so as not to affect the exposure of the substrate 1unfavorably. In addition, it is preferable that the PMMA layer issufficiently thick so as to inverse the phase of an irradiated light by180°.

A second example will be described by reference to FIG. 17, which isdifferent from the first example in that multi-stage phase shifter 8 isused.

Referring to FIG. 17(a) a substrate 1 of quartz is prepared, and atransparent layer 9 is deposited on the substrate 1. For the transparentlayer 9, a PMMA layer is spin-coated to a thickness of 0.372 μm. Then,an exposing radiation is applied through electron beams 10 at anaccelerating voltage 25 KV by the amount of exposure of about 130μC/cm². In this way a phase shifter 2 having a phase different of 180°is formed. In order to form an area whereby a phase difference of 90° isgiven between 180° and 0°, the amount of exposure applied thereto isadjusted to 50 μC/cm². In order to form two areas whereby phasedifferences of 120° and 60° are given between 180° and 0°, the amountsof exposure are set to 45 and 60 μC/cm².

In order to form an area whereby the phase difference is varied from180° to 0° continuously, it is only necessary to change the amount ofexposure in this area continuously from 0 to 130 μc/cm².

In the first example, PMMA is used for the phase shifter 2 which is anorganic layer but PMMA may involve a problem of durability; morespecifically, when in using a transparent phase-shifter mask to effect aradiating exposure, KrF excimer laser or any other far ultra-violet ray(having a wavelength of 200 nm to 300 nm) is used as a source of light,the problem is often caused because the far ultra-violet ray generateshigh energy owing to its relatively short wavelength and causes theablation of the PMMA layer. In order to solve this problem, an effectivechemical formula is shown in FIG. 18.

FIG. 18(a) shows the chemical formula of salt-onium that can be used asa photosensitive agent. FIG. 18(b) shows polysiloxane having a ladderstructure used as a base resin. The resist used in the example containsa mixture of polysiloxane of FIG. 18(b) as a base resin and a salt-omiunof FIG. 18(a) by 0 to 10 part by weight.

This resist has a high sensitivity of 0.2 μC/cm² to the electron beams,and therefore it is possible to use this resist for the phase shifterinstead of the PMMA. Thus a stable phase shifter can be produced by asimple process.

Instead of using polysiloxane for a base resin, any other substance canbe used if it consists mainly of polysiloxane having a silicon-basechain structure. Any other agent having such a base resin and having asensitivity to electron beams can be used as a resist and a phaseshifter. For such base resins having a required sensitivity,chloromethylated polysiloxane and clear polymers such as silicon resinscontaining highly reactive end groups can be used.

A third example will be described by reference to FIG. 18:

A substrate 1 of quarterback is prepared and an ITO layer is depositedon the substrate 1. A transparent layer 9 such as silicon nitride layerand an electron line resist 12 are coated on the substrate 1. For theelectron line resist 12 chloromethyl (hereinafter called CMS).

As shown in FIG. 19(a), a silicon nitride layer 9 is formed on thesubstrate 1 to a thickness of about 0.18 μm by sputtering, followed bythe spin-coating of a CMS layer 12 to a thickness of about 0.5 μm. Then,the CMS layer 12 is heated at 120° C. for 30 minutes and hardened.

The next step is to expose the desired pattern by using electron beams10 by an amount of exposure of 6 μ². Then a mixture of isopentyleacetate and ethylene glycol monoethyl ether is sprayed for a minute soas to effect development. In this way the resist pattern 13 is formed asshown in FIG. 19(b).

By using the resist pattern 13 as a mask, the silicon nitride layer 9 isdry etched by using a mixture of CF₄ and O₂ whereby the silicon nitridelayer 9 is removed, and the substrate 1 is exposed. The process so faris shown in FIG. 19(c).

The final step is to remove the resist pattern 13 with an oxygen plasma,so as to form the phase shifter 2 of the silicon nitride layer 9. Inthis way the transparent phase-shifter mask is formed as shown in FIG.19(d).

A fourth example will be described by reference to FIG. 20 in which aninorganic layer is formed by vapor deposition for a phase shifter:

A substrate 1 of quartz is prepared and a transparentelectroconductivity layer such as an ITO layer is deposited to athickness of 0.1 μm. Then, a transparent layer 9 such as a PMMA layer isspin-coated to a thickness of 0.5 μm on the substrate 1, followed by aheat treatment at 170° C. for 30 minutes whereby the PMMA layer is driedand hardened. Then, the desired pattern is exposed by using electronbeams 10 having an accelerating voltage 25 kV by an amount of exposureof 130 μC/cm². The next step is to develop it with a mixture of MIBK andIPA at a ratio of 1:2 by volume. The development dissolves the exposedarea to form a desired resist pattern 13. The substrate 1 is rinsed withIPA and spin-dried. In this way the resist pattern is formed as shown inFIG. 20(b).

The next step is to deposit a transparent layer 9 such as a siliconoxide layer to a thickness of 0.47 μm by vacuum vapor deposition usingelectron beams, thereby forming a phase shifter 2. The resist pattern 13is removed by submerging the substrate 1 in aceton, and at the same timethe silicon oxide layer on the resist pattern is also removed. However,the silicon oxide layer deposited directly on the substrate 1 remains onthe substrate 1. In this way a transparent phase-shifter mask having thephase shifter 2 of silicon oxide layer is formed.

Now, a method of applying the transparent phase-shifter mask accordingto the present invention to a semiconductor device will be described:

Take the transparent phase-shifter mask obtained as the third examplefor example. An exposing device is equipped with a 1/5-reductionprojector which has an exposing wavelength of 365 nm (i line) and anumerical aperture (NA) of lens of 0.45. The semiconductor substrate 6is coated with a resist, and is exposed to light through a transparentphase-shifter mask, followed by development. In this way a resistpattern is form on the semiconductor substrate 6.

FIG. 21(a) shows the transparent phase-shifter mask used in theabove-mentioned process, indicating the pattern of the phase shifter 2.FIG. 21(b) shows the resist pattern 7 formed on the semiconductorsubstrate 6 in the above-mentioned manner. As is evident from FIGS.21(a) and 21(b), the phase shifter 2 has stripes, hereinafter called"stripe-shaped phase shifter". The resist pattern 7 has two times asmany stripes as those of the phase shifter 2.

The process using a transparent phase-shifter mask according to thepresent invention is more advantageous than a process without using anyphase shifter, in that the former can achieve a line-and-space patternwith 0.3 μm in intervals whereas the latter produces it only with 0.45μm in intervals. Furthermore, the latter process cannot constantly formsingle patterns with fineness of below 0.5 μm. According to the formerprocess, it is possible to produce single patterns with fineness of 0.3μm constantly.

A process of applying the transparent phase-shifter mask of the presentinvention to dynamic RAMs (random access memory) will be described:

FIGS. 21(c), 21(e), and 21(g) show transparent phase-shifter masks usedfor exposure, each indicating the pattern of their phase shifters 2.FIGS. 21(d), 21(f), and 21(g) shows the resist patterns 7 formed on thesemiconductor substrate 6 exposed through a transparent phase-shiftermask. These are used as wiring patterns for the semiconductor device.

Referring to FIG. 21(c), a process of applying a transparentphase-shifter mask to the formation of resist patterns which makecontact holes will be described:

An area 14 is constituted by a square phase shifter 2 formed on thesubstrate 1 and rectangular substrate portion disposed on each side ofthe square phase shifter 2. An area 15 is constituted by a squaresubstrate portion and rectangular phase shifters 2 disposed on each sideof the square substrate portion. The shorter side of each rectangle ishalf as long as the side of the square, and it is necessary to set theshorter side of the rectangle below a resolution limit. In this way thesquare phase shifters 2 and the square substrate portions constituterepeated patterns extending in horizontal and vertical directions withlight-shielding areas disposed on both sides of the square portions. Itis not always necessary for the long side of each rectangle to be equalto the side of each square.

Of irradiated lights, a light passing through edge portions of the phaseshifters 2 effects exposure. Lights passing through the rectangularphase shifters 2 and through the rectangular substrate portions adjacentthereto have phases in reverse to each other, thereby reducing the lightintensity. This means that the rectangular phase shifter 2 and therectangular substrate portions provide a light-shielding area incombination against the irradiated light.

Without using the phase shifter 2 the resulting contact hole is only0.55 μm, whereas the transparent phase-shifter mask enables theformation of contact hole of 0.4 μm.

FIG. 21(e) shows a transparent phase-shifter mask used for forming apattern which becomes a capacitor in which rectangular phase shifters 2and rectangular substrate portions are formed in repeated patterns. FIG.21(f) shows the resist pattern formed by using the photomask of FIG.21(e).

In order to form a pattern for capacitor, a negative resist is used.Reversely to the examples described above, the exposed area remains as aresist pattern 7 on the semiconductor substrate 6. That is, an areawhere the light intensity is reduced is developed.

Under the conventional exposure using no transparent phase-shifter maskit is difficult to form elongated rectangular resist patterns withintervals of 0.5 μm or less. When the transparent phase-shifter mask isused, patterns having intervals of 0.2 μm can be formed.

FIG. 21(g) shows a transparent phase-shifter mask used for forming apattern allowing the formation of an element separating areas. Thispattern includes a step-shaped area and a void-present area whichincludes a space at the center. In addition, the substrate 1 includes astepped area in the substrate, and a stepped area in the pattern. In thevoid-present phase shifter portion and the stepped substrate portion,the phase shifter portion adjacent to the space, and the substrateportion adjacent to the phase shifter have narrow widths. It isnecessary to reduce the size of these narrow areas so as to be below aresolution limit.

These areas act as a light-shielder against an irradiated light. Theresist patterns 7 to be transferred to the negative resist are shown inFIG. 21(h) where the pattern is formed by the removal of the resist inthe non-exposed area.

Under the conventional method using no transparent phase-shifter mask itis impossible to secure intervals of 0.5 μm or less between theresulting resist pattern and an adjacent resist pattern. In contrast,under the method using a transparent phase-shifter mask of the presentinvention the pattern intervals can be reduced to 0.2 μm.

There are other forms of transparent phase-shifter mask for forming theresist pattern 7 of FIG. 21(h).

In order to form patterns by using the transmissive phase-shifter mask,various mask patterns can be prepared depending upon the patterns to beformed. Examples are shown in FIGS. 21(i) and 21(j).

FIG. 21(g), as referred to above, in the void-present phase shifterportion and the stepped substrate portion, the phase shifter portionadjacent to the space, and the substrate portion adjacent to the phaseshifter have narrow widths. It is necessary to reduce the size of thesenarrow areas so as to be below a resolution limit.

The patterns shown in FIGS. 21(g), 21(i), and 21(j) can be selected inaccordance with a desired shape of the light-shielding areas where thepattern is formed below a resolution limit.

As shown in FIG. 21(g), an area is provided where the phase shifter 2 isformed in the substrate 1 and an area where a void is formed in thephase shifter 2, thereby forming a pattern incapable of resolution. Asshown in FIG. 21(i), an area is provided where a phase shifter portionis formed in the area of the substrate 1 in which the phase shifter 2 isformed in the substrate 1 and where the substrate portion is formed inthe phase shifter 2, thereby forming a pattern incapable of resolution.

In FIG. 21(i) the phase shifter 2 includes areas having two side lines.Each side line of each area is parallel to an adjacent side line of anadjacent area. If the interval between one side line and an adjacentside line is below a resolution limit, this area acts as alight-shielder against an irradiated light. Likewise, if the intervalsbetween the phase shifter 2 and the substrate portion is below aresolution limit, this area acts as a light shielder against anirradiated light.

Now, referring to FIG. 22, a method of applying a transparentphase-shifter mask to the fabrication of ROMs (read only memory) will bedescribed:

FIG. 22(a) shows a transparent phase-shifter mask in which astripe-shaped phase shifter 2 is formed on the substrate 1. Thelight-intensity is reduced by edge portions of the phase shifter 2. Whenthis type of transparent phase-shifter mask is used to transfer anegative resist pattern, such a resist pattern as shown in FIG. 22(b) islikely to remain on the semiconductor substrate 6. This resist patternis used as patterns for a polysilicon gate. This resist pattern hasintervals of about 0.2 μm between the lines.

Under the conventional method using no transparent phase-shifter mask itis impossible to form such intricate pattern by a single exposure but atleast exposure must be carried out twice with the use of two photomasks.

Under the method using the transparent phase-shifter mask a singleexposure is sufficient. This simplifies the process for fabricating agate.

Referring to FIG. 23(a), an example for applying a resist pattern to aCCD solid image pickup device will be described:

A mask used for this purpose is shown in FIG. 23(a). The substrate 1 isprovided with two areas 16 and 17; the area 16 is provided by a phaseshifter 2 taking the form of a rectangle, and the other area 17 isburied partly in the phase shifter area 16.

The area 17 is formed by a repetition of phase shifter portionsincapable of resolution. When a light is radiated, the light intensityin the area 16 is reduced by each side of the rectangular shape. When anegative resist is used, the resist along the edges of the area 16 isreduced. The other area 17 acts as a light-shielder against anirradiated light, thereby making a portion devoid of a resist.

FIG. 23(b) shows a state where the a pattern described above istransferred onto a negative resist by the use of a transparentphase-shifter mask. The area 16 forms a photoelectric converter for aCCD solid image pickup device, and the area 17 forms a vertical transferchannel for reading out an electric charge stored by the photoelectricconverter. Intricate fine lines are formed in the vertical transferchannel by the edge portions of the area 16 but such fine lines do notunfavorably affect the characteristics of the vertical transfer channel.If there is a worry about any unfavorable influence, the provision ofthe multi-stage phase shifter along the longer sides of the rectangulararea 16 will eliminated the possibility of forming fine lines in thetransfer channel.

In this way, the fine light-shielding areas along the phase shifter, andthe repetition of phase shifter patterns are utilized. The intervalsbetween the resist patterns are about 0.2 μm at minimum, and in areaswhere the intervals are wider, the repetition of patterns having a widthbelow a resolution limit is used so as simplify the step for fabricatinggates.

Referring to FIG. 24(a), a method of applying the transparentphase-shifter mask to the formation of intricate gates.

An area 18 is where a take-out electrode 19 is where a gate is formed.The area 18 is constituted by intricate phase shifter portions 2a, andrectangular phase shifter portions 2b have widths below the resolutionlimit, thereby becoming a light-shielder against an irradiated light. Inthe rectangular phase shifter portions 2b the light intensity is reducedon the edge portions, thereby enabling a straight gate to be formedalong the edges of the phase shifter portions 2b. A resist patternshould not be formed along the three sides other than the side alongwhich the gate is formed. In order to prevent a gate from being formedalong these three sides, multi-stage phase shifters 8 are preferablyformed along them.

By using this transparent phase-shifter mask a resist pattern is formedby exposure on the semiconductor substrate 6. A negative resist is usedso that the area 18 and the rectangular phase shifter potions 2b formingthe gate include a light-shielder against an irradiated light. Thelight-shielding portions get rid of the resist after exposure anddevelopment.

Redundant patterns formed along the phase shifter 2 are negated by usingmulti-stage phase shifter 8. A large-size electrode to the gate isformed by using the pattern of the phase shifter 2 below a resolutionlimit. This type of transparent phase-shifter mask enables thefabrication of a semiconductor device of GaAs having a gate width of0.15 μm. Under the conventional method an electron beam drawing processwas used to form intricate fine patterns, but because of the necessityof using vacuum, the process was not suitable for the mass-production.

In the illustrated examples, PMMA, siloxane-polymer base resists,silicon nitride layers and silicon oxide layers are used but magnesiumfluoride, titanium dioxide, titanium nitride or any other transparentpolymers can be used.

In the illustrated examples, reference is made to the phase shifter usedin an exposing device having a wavelength for exposure of 365 nm but anyother exposing device can be used if the following equation issatisfied:

    t=λ/2·(n-1)                                (2)

where t is the thickness of the phase shifter, and n is a diffractionindex of the phase shifter.

In the illustrated examples the phase shifter mask consists of patternsformed a transparent photomask substrate for a transparent phaseshifter.

FIG. 25 shows a photomask having a light-shielding area using a phaseshifter and a light-shielding layer such as a chromium layer incombination. The resulting light-shielding pattern will be described byreferring to FIG. 25:

In FIG. 25 an electrode to a gate is formed in an area 28, and the gateis formed in an area 29. The area 28 is a light-shielding layer formedby chromium layer or molybdenum silicide.

A rectangular phase shifter 2 has an edge where the light intensity isreduced, thereby forming a straight gate along the edge. No resistpattern should be formed along the three sides other than the side alongwhich the gate is formed. In order to achieve it, multi-stage phaseshifters 8 are formed along them.

By using this type of transparent phase-shifter mask, a resist patternis formed by exposure on the semiconductor substrate 6. A negativeresist is used so that the area 28 and the rectangular phase shifter 2forming the gate include a light-shielder against an irradiated light.The light-shielding portions get rid of the resist after exposure anddevelopment.

Redundant patterns formed along the phase shifter 2 are negated by usingmulti-stage phase shifter 8. A large-size electrode to the gate isformed by using the pattern of the phase shifter 2 below a resolutionlimit. This type of transparent phase-shifter mask enables thefabrication of a semiconductor device of GaAs having a gate width of0.15 μm. Under the conventional method an electron beam drawing processwas used to form intricate fine patterns, but because of the necessityof using a vacuum, the process was not suitable for mass-production.

Instead of using a transparent phase-shifter mask for a phase shiftmask, it is possible to use a phase shifter and a light-shielding layersuch as a chromium layer in combination.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty that reside inthe present invention, including all features that would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. A process for forming a photomask, the processcomprising the steps of depositing a transparent layer on a transparentsubstrate to a sufficient thickness to give a phase difference of 180°multiplied by an odd number to incident light, coating the transparentlayer with a resist, and effecting the exposure of part of the peripheryof the resist by a light whose focus is deviated to form a phase-shifterwhose one side diminishes in thickness step by step.